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SHEET METALDESIGN GUIDE.An Overview of Materials, Features, Tolerances and Finishing forLow-Volume Prototypes to End-Use Applications.w w w.GoProto.com1 - 8 8 8 - G o P r o t o (467-7686) quotes @GoProto.com

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OverviewGoProto specializes in quick-turn, low quantity prototype through production sheet metal parts andweldments. Our goal is to make you the best parts with the quickest lead times at the lowest price possible.It is estimated that manufacturers can spend as much as 50% of our time addressing design errors thataffect manufacturability. The reason for this is often due to the wide gap between how sheet metal partsare designed in CAD and how they are actually fabricated on the shop floor. Also specifics on material type,thicknesses, possible tolerances and shop-specific cutting and fabrication techniques may be lacking.What does that mean for you? Extra cost, longer lead time and parts that might not suit your needs. Thepurpose of this GoProto Sheet Metal Design Guide is to help you define the specifics so that your designsare ready to go in production with us in such a way that theoretical versus actual is minimal and yourfeatures match available processes. This will help us make your parts fast, make them right and makethem inexpensively.Because all parts fabricated with sheet metal start out as sheet, we suggest starting with a flat pattern inCAD that can be folded in your design package. We request that you submit your CAD files in the foldedform – we’ll work on flattening it. Because all sheet metal stretches as it is formed, receiving your final partallows us to do some work backward to extrapolate just where to put your features in the flat pattern byallowing for our bend radii needed and the cutting and fabrication techniques we will use.Thank you for giving us the opportunity to work with you. If you have any questions after reading throughthis guide, please let us know!3

Material SelectionWhen designing with sheet metal, there isa relationship between the design of thepart, the use of the part and the choice ofmaterial.While the design can guide you to specificmaterials, the materials themselves canoften lead to functionality and cosmeticimprovements based on performancecharacteristics of the chosen metal alloy.Further, the material choice will impact thedesign integrity as well as affect:Cost, Labor & Tooling Requirements4

Material SelectionALUMINUMAluminum is both temperature resistant and weldable in most forms. It is a great general purpose material. It is also corrosion resistant when treated and hasa high strength-to-weight ration at a moderate price point. The most common aluminum we see for sheet metal forming is AL5052, but we can also work withAL6061 for flat patterns.GALVANIZED STEELGalvanized steel is steel coated with zinc during a zinc solution heat batch. This process protects the steel from corrosion and cracks and, like aluminum, has agood strength-to-weight ratio. Galvanized steel has a lower cost per pound than aluminum but may have some issues in adhesion of paints depending on enduse. Common used are chassis for electronics printers and medical equipment.GALVANEALED STEELSimilar to galvanized steel, galvanealed steel has been taken one step further and heat treated after the initial zinc coating. While this process is moreexpensive, it adds paintability and surface aesthetics to the list of advantages.STAINLESS STEEL SHEET IS AVAILABLE IN TYPES 301, 304, 316 and 430Type 301 is excellent for high strength and corrosion resistance. Type 304 can be easily roll-formed or bent, and its excellent corrosion resistance andweldability make it one of the most popular grades. Type 316 is a high corrosion resistance alloy, providing greater resistance to pitting-type corrosion. Typicaluses for stainless steels include marine, chemical, paper, textile, and food service applications. Type 430 is a ferritic stainless steel with excellent corrosionresistance. This grade does not work harden rapidly and can be formed using both mild stretch forming, bending, or drawing operations. This grade is used in avariety of interior and exterior cosmetic applications where corrosion resistance is more important than strength. Type 430 has poor weldability compared tomost stainless steels due to the higher carbon content and lack of stabilizing elements for this grade, which requires post weld heat treatment to restore thecorrosion resistance and ductility.COPPERDepending on the end use of the designed part, copper sheets have very favorable performance characteristics. It has a high thermal and electrical conductivity and is resistant to corrosion. Copper is also ductile and malleable and is both anti-bacterial and biostatic. We typically use C1100 for sheet metal forming.BRASSAn alloy of copper and zinc, brass is very ductile and corrosion resistant. Brass is strong and the hot versus cold working characteristics can be varieddepending on the quantity of zinc in the alloy. This gives brass sheet a wider range of performance depending on end use of the designed part. We typicallyuse C2680 for sheet metal forming.LOW CARBON STEELLow carbon steel contains 0.05-0.32% carbon compared to medium, high and ultra-high carbon steel. This makes low carbon steel a more cost-effective choice.It is also malleable and ductile.OTHERSAlthough the above materials are most commonly used, we can also work with less common materials such as: Tinplate SPTE (thin steel coated with tin),Beryllium Copper (C1720), Nickel Silver (C7521), Phosphor Bronze (C5191), Spring Steel (SK7 and SK5), and Hot-Rolled Steel (SPHC).But if you don’t see your material listed, just ask!5

Material SelectionRecommended Thicknesses for Common Sheet Metal Alloys:AlloyMinimum (in.)Maximum (in.)Al 50520.0120.126Brass C26800.0080.157Copper (C1100)0.0120.157Stainless Steel0.0030.315Cold Rolled Carbon Steels ---Commercial Quality (SPCC)0.0200.126Electrogalvanized, Zinc Plated (SECC)0.0160.079Hot Dip-Galvanized (SGCC)0.0200.157Tin-Plated (SPTE)0.0060.031Beryllium Copper (C1720)0.0030.020Nickel Silver (C7521)0.0040.024Phosphor Bronze (C5191)0.0030.079Spring Steel (SK7), Un-treated0.0080.236Spring Steel (SK7), Heat-treated0.0240.063Hold Rolled Steel (SPHC)0.0470.236Considerations for Material SelectionIn choosing the best material for your design, end use and performance characteristics of the finalproduct should be considered as well. Some factors to consider include: Strength requirements of final part Aesthetics Geometry Joining Corrosion Requirements Desired Weight FormabilityMaterial selection is not only important to cost, it is intricately tied to the design of the part and canhelp or hurt the part’s integrity depending on material characteristics.6

FeaturesDetermine how to best prepare for the following:BENDSThe most common sheet metal form is a bend. Bends can give strength and shape to a part and are formed in a machine using bend brakes. Since sheet metalcannot be bent to a 90-degree position without breaking at sharp corners, all bends will have an acceptable bend radius. Bends in the same plane should bedesigned in the same direction. Our preferred bend radius is 0.030”.HEMSHems are folds at the edges of a part, or wall, often resembling a U or C shape. Hems can be open or closed. However, overall tolerances will depend on theradius, sheet thickness and any other features near the hem. They are more difficult to produce with prototype tooling and may require multiple bend steps.Hems improve part strength, straightness and eliminate sharp edges.OFFSETSOffsets are the creation of Z shaped bends on a part. Other offsets can be introduced, but it is important to maintain the minimum distance between featuresand in distance between parallel surfaces.HOLES and SLOTSHoles and slots are locations where joining devices such as bolts, tabs and other features keep the part in place. Location relative to the edge is critical asholes near the edge can cause issues with deburring and within the machine during processing can cause wear on machine tooling. It is best to specify holediameters greater than the sheets thickness. Hole diameters less than the sheet thickness results in higher punch loading, longer burnish in the holes andexcessive burr. It can also lead to slug-pulling when withdrawing the punch.In a similar vein, the spacing between holes should be at least two times the sheet thickness to ensure strength of the metal and prevent hole deformationduring bending or forming. Finally, holes should be at least the sheet thickness from the edge and spaces between holes and the bend should be 1.5 times thesheet thickness plus the bend radius.NOTCHES, LUGS and TABSTaking note of the grain structure is critical to avoid cracks with lugs or tabs that are cut on three sides and bent out. For instance, lugs formed parallel to thegrain direction usually tend to form cracks. The recommended practice is to for lugs perpendicular or at an angle of 45 degrees towards the grain direction.BEND RELIEFBend relief is added to prevent cracking and tearing that can occur in complicated bends or bends with harsh radii. This provide stress relief to the part,prevents the spring-back effect, and adds stiffness to the final part.CHAMFERSSome finished parts may have a sharp 90-degree or near 90-degree corner. Chamfering is a feature whereby the sharpest corners are rounded off to preventcatching, snagging or injury. Chamfers at corners and beads on bends increase the stiffness.COINING and COLLARSCollars increase the stiffness around pierced areas such as holes and slots. Coining and embossing around flared holes improve strength and the likelihood ofmaintaining flatness.WELDINGGoProto can use welds to join parts together, finish corners, fabricate features in prototype, tack hardware in place or keep hems and bends folded. We canweld most of the materials listed but types of weld and finish on welds can affect tolerances. Because this is such a widely varying technique, tolerances dovary and we’ll work to adhere to your drawings and can work with you through any questions.7

FeaturesChemical Etching:In some cases on small or irregular-shaped holes,GoProto may use chemical etching.Chemical Etching is a process that producesburr-free holes of any shape, down to 0.004” indiameter. No mechanical force or heat is used, sothe material properties are unaltered and free ofstresses. It’s ideal for creating perforated metalproducts, such as screens, meshes, grids, filters,and so on.Almost any material can be etched, includingthose considered hard to machine.Because no hard tooling is required, design iterations arelow-cost and fast.8

Tolerances & GuidelinesIf parts are designed properly, GoProto will maintain industry standard or better tolerances asdescribed below.General linear tolerances are /- .005” for mostfeatures on parts under 12”. Over 12” tolerancesare /- .010”.We follow general tolerances to ISO-2768-M andthey include:BENDSBends are formed in a machine using bend brakes with a standard tolerance is /- 1 degree. It is also recommended that the bendradii not be less than the thickness of the sheet (T).HEMSMinimum inside diameter on a hem should be 4X the thickness of the sheet. However, overall tolerances will depend on the radius,sheet thickness and any other features near the hem.OFFSETSOverall, all bend radii in offsets should be 0.030”. The distance between parallel planes should be at least 2X the thickness of thesheet. A tolerance of /- 0.012” between features is recommended. Other offsets can be introduced, but it is important to maintainthe minimum distance between features and in distance between parallel surfaces.HOLES and SLOTSHoles and slots should be at least one material thickness in diameter (T) and preferably 4T away from the edge of the material. Ifusing inserts, use the manufacturer’s specifications to determine distance from the edge.NOTCHES and TABSNotches and tabs should be at least 1T or 0.04” , whichever is greatest. They should also be no more than 5 X the width.BEND RELIEFBend relief should be no deeper than 1T plus the bend radius and should not be wider than 0.030”.9

TolerancesThe difference between the theoretical world of sheet metal design and actual:TheoreticalActualHole diameters, spacing, and tolerancevalues are exact.Cutting method has a large effect on tolerances.Actual designs vary after bending, whichcan lead to misalignments.No need for ribs, collars, or chamfers.Stiffness decreases in pierced areas or largeopen surface areas.No need for coining, embossing, or beads.Strength decreases and we are unable tomaintain flatness.Lugs without understanding the grainstructure of the metal is no problem.Lugs parallel to the grain structure maylead to crack formation.Interpreting DesignsTolerances can be different on your actual part when compared to what i